Fix keyring file missing after Vault restart

    Vault can be killed by signal SIGTERM in anytime. If Vault is
    doing keyring rotation and the fd is still not closed at that time,
    the size of the keyring file will be zero. The orignal key will be
    lost totally.
    Because the file is opened with os.O_TRUNC, and the fd will be closed
    automatically after Vault exits.
    Then try to unseal Vault after it startup again, the keyring file will
    be deleted in getInternal function due to its size is zero. So that is
    why the keyring file is missing. And the Vault cannot be unsealed
    anymore due to the file missing.
    Write data into a temp file then move it can avoid the file crash.

    Fix:hashicorp#15680

Create 15946.txt